Steerable three-dimensional magnetic field sensor system for detection and classification of metal targets

ABSTRACT

A steerable electromagnetic induction (EMI) sensor system for measuring the magnetic polarizability tensor of a metal target. Instead of creating a vertical magnetic field from a horizontal loop transmitter configuration used by most prior art EMI metal detectors, the transmitter geometry of the sensor system&#39;s antenna is designed especially for creating multiple horizontal and vertical magnetic fields and for steering the same in all directions. The horizontal magnetic field (HMF) antenna has the potential advantage of a relatively uniform magnetic field over a large volume. A second potential advantage of the HMF antenna is that compared to a conventional loop antenna, the magnetic field intensity falls off slowly with distance from the plane of the antenna. Combining two HMF sensor systems creates a sterrable two-dimensional magnetic field sensor. Combining the steerable HMF sensor with a vertical magnetic field antenna forms a three-dimensional steerable magnetic field sensor system.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to detecting and identifying metaltargets in general and, more particularly, to a steerablethree-dimensional magnetic field sensor system and method for detectingand identifying metal targets, such as unexploded ordnance (UXO),underground utilities, high metal content landmines and low metalcontent landmines buried in the soil (or visually obscured) based on theelectromagnetic response of the target to a time-domain wide bandwidthelectromagnetic spectrum.

[0003] 2. Description of the Related Art

[0004] Most electromagnetic induction (EMI) metal detectors use a loopantenna to create a magnetic field in the vicinity of a metal target forthe purposes of detection and identification. One of the most importantfunctions of a magnetic field antenna is to project a strong magneticfield at the site of the target.

[0005] Typical loop antennas are formed of multiple turns of wire arounda central axis. The magnetic field strength of a loop antenna is astrong function of distance from the antenna. Far from the antenna,along the axis of the loop antenna, the field strength variesapproximately as 1/r³, where r is the distance from the plane of theloop to the object, Off-axis, the antenna field strength and directiontends to be a very complex function of position, with the fieldintensity very strong near the wires in the loop and weaker near thecenter of the loop.

[0006] One of the consequences of the loop antenna's complex spatialfield strength is the fact that a metal target is excited with a complexmagnetic field. When a buried target of unknown depth is scanned with anEMI sensor, the spatial distribution of the excitation field at thetarget is not known. Some target identification algorithms assume thatthe target is excited with a uniform field. If the field is in factcomplex, the target's time or frequency response to the field is notwell characterized. This may tend to complicate or confound a targetidentification algorithm.

[0007] In addition, with the target at the center of the loop, the loopmagnetic field antenna only measures the vertical component of atarget's decay response.

[0008] A metal target can be modeled by defining a magneticpolarizability tensor: $\overset{\_}{M} = \begin{pmatrix}{M_{x}(t)} & 0 & 0 \\0 & {M_{y}(t)} & 0 \\0 & 0 & {M_{z}(t)}\end{pmatrix}$

[0009] where the diagonal components of the tensor are the timeresponses of the target to excitations in an orthogonal reference framecentered on the target. Models of this nature generally assume that theexcitation field strength is uniform over the target's volume. For aloop antenna oriented directly over a target, the antenna only excitesthe vertical component of the target's time decay response, M_(z)(t).For accurate target classification, it is necessary to measure all threecomponents of a target's magnetic polarizability tensor.

[0010] Accordingly, a need exists to develop a magnetic field sensorsystem that can project a strong magnetic field deeply into the ground;excite the target with a uniform magnetic field; and measure thethree-dimensional components of the target's magnetic polarizabilitytensor. As noted above, prior art EMI metal detectors that use loopmagnetic field antennas do not address all of these issues.

SUMMARY OF THE INVENTION

[0011] The present invention provides a steerable three-dimensional(3-D) magnetic field sensor system for detection and classification ofhidden or obscured metal targets, as well as voids in soil. Thesteerable 3-D magnetic field sensor system measures the horizontal andvertical components of a metal target's eddy current time decaysignature. Instead of creating a vertical magnetic field from ahorizontal loop transmitter configuration used by most prior art EMImetal detectors, the transmitter geometry of the sensor system's antennais designed for creating horizontal and vertical magnetic fields and forsteering the same. Two horizontal magnetic field (HMF) antennas and avertical loop electromagnetic field antenna are combined to form thesteerable 3-D magnetic field sensor system.

[0012] One of the potential advantages of the steerable 3-D magneticfield sensor system is the relatively uniform magnetic field that iscreated over a large volume by the HMF antennas. A second potentialadvantage of the EMI sensor system is that compared to a conventionalloop antenna, the magnetic field intensity falls off slowly withdistance from the plane of the antenna. These two advantages potentiallymake the steerable 3-D magnetic field sensor system well suited fordetection and classification of metal targets buried deeply in theground (e.g., landmines, unexploded ordnance (UXO) and undergroundutilities).

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a diagram illustrating the antenna geometry of ahorizontal magnetic field (HMF) antenna of the steerable electromagneticinduction (EMI) sensor system according to the present invention;

[0014]FIG. 2 is a chart showing Bx as a function of x at the center ofthe HMF antenna for different distances from the plane of the HMFantenna in the z direction;

[0015]FIG. 3 is a chart showing the angle of Bx as a function of x fordifferent heights above the plane of the HMF antenna;

[0016]FIG. 4 is a log-linear chart that compares Bx from HMF antennaaccording to the present invention to Bz of a prior art loop antennaversus distance from the plane of the HMF antenna;

[0017]FIG. 5 illustrates a three-dimensional plot of Bx surrounding theHMF antenna of the present invention;

[0018]FIGS. 6A and 6B illustrate the field distribution from the priorart loop antenna and the HMF antenna of the present invention,respectively;

[0019]FIG. 7 is a block diagram of the HMF antenna according to thepresent invention;

[0020]FIG. 8 is a diagram of an experimental setup for performing testtarget measurements using the HMF antenna of the present invention;

[0021]FIG. 9 is a diagram illustrating the HMF antenna having twistedreturn wires, twisted parallel wires and damping resistors;

[0022]FIG. 10 is a chart showing time decay response data from acalibration test over a time period;

[0023]FIG. 11 is a log-log chart of data from an aluminum soda can and aVal 59 metal anti-personnel (AP) mine taken approximately 15 cm abovethe HMF antenna;

[0024]FIG. 12A is a diagram illustrating operation of the HMF antennawhere receiver units are vertically oriented;

[0025]FIG. 12B is a diagram illustrating a vertical receiver arrayconfiguration for the receiver units of FIG. 12A;

[0026]FIG. 13A is a diagram illustrating operation of the HMF antennawhere receiver units are horizontally oriented;

[0027]FIG. 13B is a diagram illustrating a horizontal receiver arrayconfiguration for the receiver units of FIG. 13A;

[0028]FIG. 14 is a diagram illustrating the horizontal receiver arrayconfiguration, the HMF antenna and a differential amplifier;

[0029]FIG. 15A is a diagram illustrating detection of a metal targetusing the HMF antenna and the horizontal receiver array configuration;

[0030]FIG. 15B is a diagram illustrating detection of an undergroundvoid using the HMF antenna and the horizontal receiver arrayconfiguration;

[0031]FIG. 16 is a diagram illustrating two HMF antennas at right anglesto each other forming a two-dimensional HMF antenna that can generate ahorizontal magnetic field which can be steered in any direction;

[0032]FIG. 17 is a diagram illustrating a steerable magnetic fieldsensor system having the two-dimensional HMF antenna; and

[0033]FIG. 18 is a diagram illustrating a steerable magnetic fieldsensor system having the three-dimensional HMF antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] A discussion is first made as to the underlying physics utilizedby a horizontal magnetic field (HMF) sensor system of the presentinvention. Following this discussion, a description is provided of thesteerable two-dimensional HMF sensor system followed by a discussion ofexperimental data demonstrating the sensor system's capabilities.Following this discussion, a description is provided of the steerable3-D magnetic field sensor system which includes two HMF antennas and avertical loop electromagnetic antenna.

[0035] I. HMF Antenna Model

[0036] The innovative invention of the HMF antenna has not beenpreviously discussed as has the vertical magnetic field antenna. Assuch, it is instructive to review the basic physics of the innovativeHMF antenna so that the advantages of such a magnetic field antenna andmagnetic field receivers can be appreciated.

[0037] Reviewing a basic physics textbook, one of the first geometries astudent is asked to solve is the “sheet current” problem. The textbookproblem and its solution very clearly describes the present antenna'sconfiguration. The problem: “Long, straight conductors with squarecross-section and each carrying current I are laid side by side to forman infinite current sheet. The conductors lie in the xy-plane, areparallel to the y-axis, and carry current in the +y direction. There aren conductors per meter of length measured long the x-axis.” For aninfinite conducting sheet, the field is in the x direction and there isno magnetic field variation in the z direction; the field is constantand is given by:

B=μ ₀ n I/2  (1)

[0038] where I is the current in the wire and n is the number of wiresper meter of length measured along the x-axis.

[0039] Expressed another way, the sheet current is a horizontal magneticfield (HMF) generator or antenna. The important feature of Equation (1)is the fact that the magnetic field is constant with z, the distancefrom the plane of the HMF antenna. Equation (1) forms the basis of thepresent magnetic field antenna. The objective is to create anapproximation to an infinite sheet current and the magnetic field willhave a relatively uniform shape and a slow magnetic intensity fall-offwith distance from the plane of the HMF antenna. Additionally, theunique character of this HMF allows the present invention to use uniquemagnetic field receiver configurations that enhance the time-domainperformance of the sensor system compared to a conventional loop EMIsensor system.

[0040] Preliminary HMF antenna modeling uses the simplified geometry ofFIG. 1. Using the Biot-Savart Law, the approximate x and z components ofthe magnetic field can be written as: $\begin{matrix}{{{B_{x} = {\frac{\mu_{0}I\quad {z0}}{4\pi}{\sum\limits_{n = 0}^{N}( {\lbrack {{z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}} \rbrack^{- 1}\lbrack {\frac{( {L - {y0}} )}{\sqrt{( {L - {y0}} )^{2} + {z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}}} + \frac{y0}{\sqrt{( {{y0}^{2} + {z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}} }}} \rbrack} )}}}{B_{z} = {\frac{\mu_{0}I}{4\pi}{\sum\limits_{n = 0}^{N}( {{( {{x0} - {n\quad \Delta \quad x}} )\lbrack {{z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}} \rbrack}^{- 1}\quad \quad\lbrack {\frac{( {L - {y0}} )}{\sqrt{( {L - {y0}} )^{2} + {z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}}} + \frac{y0}{\sqrt{( {{y0}^{2} + {z0}^{2} + ( {{x0} - {n\quad \Delta \quad x}} )^{2}} }}} \rbrack} )}}}}\quad} & (2)\end{matrix}$

[0041] where N is the number of current elements (wires) carryingcurrent I, Δx is the wire separation, L is the length of the HMFantenna, and X0, Y0 and Z0 are the location of the magnetic fieldmeasurement points in space. For this preliminary modeling effort, thereturn path of the current is ignored. The current return path isimportant and when constructing the actual HMF antenna, the returncurrent path can be placed at a relatively large distance to the nominaldetection area. The true Bx component of the magnetic field will beslightly distorted from the values generated by the presentcalculations. The magnetic field distorting caused by the actual returnpath of the wires can be controlled for the desired field uniformity bymoving the wires far from the detection area or by adding magneticshielding to the return path wires. In any case, the magnetic fielddistortions are an order of magnitude smaller than the spatial fielddistortion of a loop antenna. In addition, with the simple geometry of aplane set of wires, there is no By component of the magnetic field.

[0042] Calculations using Equation (2) were made with the followingantenna parameters: X=100 cm, L=300 cm, N=50 and Δx=2 cm. Theseparameters were selected for a conceptual application of a HMF sensormounted on a UXO survey cart similar to the United States Navy'sMulti-sensor Towed Away Detection System (MTADS). MTADS uses three EMmetal detectors, with 1 m diameter loop antennas to cover a 3 meter widesearch area. The MTADS is designed to search and identify buried UXO.

[0043]FIG. 2 is a plot of Bx versus x at the center of the HMF antenna(y=150 cm) for different distances from the plane of the HMF antenna inthe z direction. Note that the field intensity is relatively uniformexcept close to the edge of the HMF antenna. For a particularapplication depth, the HMF antenna parameters can be adjusted for thedesired Bx field uniformity.

[0044]FIG. 3 shows the angle of Bx as a function of x (cross antennatrack). It is noted that if a receiver coil is placed at the center ofthe HMF antenna in the plane of the HMF antenna, there is no net fluxthrough the receiver coil. The Bz components of the magnetic fieldcancel. This implies that a horizontal receiver coil so placed will, tofirst order, not “see” the turn-off transients of a pulsed time-domainversion of the sensor. This also implies that a HMF antenna couldpotentially be used in a frequency domain sensor system, since thecoupling between the transmitter and receiver are minimized.

[0045]FIG. 4 is a log-linear plot that compares Bx from the HMF antenna(1 m by 3 m) to Bz of a prior art loop antenna (1 m diameter) versusdistance from the plane of the HMF antenna of the present invention. Themagnetic fields from each antenna have been normalized to 1 at a depthof 10 cm to show the relative field intensity fall-off with distance.The calculations were made along the centerline of each antenna.

[0046] Over the depth range of 10 cm to 500 cm, FIG. 4 shows that theHMF antenna field strength varies by approximately a factor of 30, whileover the same distance range, the loop antenna varies by a factor of1000. Also shown in FIG. 4 is a third curve of a HMF antenna with areturn current path 1 m away from the primary antenna surface. The Bxfield strength is lower than the HMF field without the return path, butthe field still falls off more slowly than the loop antenna. Over thedistance range of 10 cm to 500 cm, the HMF antenna field strength with areturn path included varies by approximately a factor of 60. This isstill much less than the prior art loop antenna value of 1000.Increasing the return path separation distance or using some form ofmagnetic shielding will reduce the return path effect even more.

[0047]FIG. 5 shows a three-dimensional (3-D) plot of Bx surrounding theHMF antenna. Note that the field in both the x and y directions isrelatively uniform, except near the current carrying wires and edges ofthe HMF antenna. As one would expect, the field near the wires is veryintense, causing local “hot spots.”

[0048] Another way to view the field distribution from the loop and HMFantennas is shown in FIGS. 6A and 6B. Here, we have a simplifiedconceptual diagram of the magnetic field vectors from the differentantennas. FIG. 6A shows the magnetic field surrounding a simple loopantenna looking from the side. The field is relatively uniform in thecenter of the antenna and is oriented in the vertical direction. As wemove away from the plane of the loop in the −z direction, the fieldcontinues to be vertical along the axis, but off-axis the field has amore complex shape. The field is approximately horizontal under the loopconductors.

[0049]FIG. 6B shows the magnetic field surrounding the HMF antennalooking down the current carrying wires. The current carrying wires areperpendicular to the plane of the paper and the current direction is outof the paper. In the region near the center of the HMF antenna, themagnetic field is in the horizontal direction. As one moves from thecenter of the HMF antenna, the field becomes more complex, with Bzstarting to dominate at the edge. However, as FIGS. 2 and 5 show, thefield remains relatively uniform in the x direction.

[0050] II. Sensor System Description

[0051] The steerable 3-D magnetic field sensor system is composed ofthree subsystem components: (1) two identical time-domain HMFelectromagnetic sensor subsystems, each producing a HMF at right anglesto each other; and (2) a conventional horizontal loop antenna magneticfield sensor system that produces a vertical magnetic field. Each HMFsubsystem in turn is composed of two basic components: (1) the HMFantenna and (2) the magnetic field receiver.

[0052] A magnetic field is a vector quantity. It has magnitude anddirection and can be decomposed into individual components in anorthogonal coordinate system. For example, in a Cartesian coordinatesystem of X, Y and Z, a magnetic field vector can be represented by Bx,By and Bz. If we generate in the same volume (spatial region) a magneticfield in the X direction, Bx; generate a magnetic field in the Ydirection, By; and generate a magnetic field in the Z direction, Bz, theindividual magnetic field components combine to form a new magneticfield that has a magnitude and direction given by:${\overset{->}{B}} = \sqrt{B_{x}^{2} + B_{y}^{2} + B_{Z}^{2}}$$\theta = {\tan^{- 1}\lbrack \frac{B_{y}}{B_{X}} \rbrack}$$\delta = {\tan^{- 1}\lbrack \frac{B_{z}}{\sqrt{B_{x}^{2} + B_{y}^{2}}} \rbrack}$

[0053] where θ is the angle measured from the x axis in the XY plane andδ is the angle from the XY plane to the B vector.

[0054] By varying the magnitude of the three magnetic field components,a magnetic field can be projected in any direction. A X directionmagnetic field is generated by a X directed HMF antenna, a Y directionmagnetic field is generated by a Y directed HMF antenna, and a Zdirection magnetic field is generated by a Z directed horizontal loopantenna. Combining all three antennas, a 3-D magnetic field sensorsystem of the present invention is created.

[0055] The details of the HMF subsystem is described first. Later, theuse of two HMF subsystems and the vertical magnetic field sensor(horizontal loop) will be described that show the creation of thesteerable 3-D magnetic field sensor system.

[0056]FIG. 7 is a simplified block diagram of the HMF sensor systemdesignated by reference numeral 100 according to the present invention.An approximation to a sheet current is created by a series of closelyspaced parallel wires 102. These wires form the active surface of theHMF antenna 104. The closely spaced parallel wires 102 are connected tohigh-speed electronic switches (E-Switch) 106.

[0057] Return current wire segments 108 of wires 102 exit the E-Switches106 in a direction perpendicular to the plane of the HMF antenna 104.The circuit is completed with a set of parallel return wire segments 110relatively far away from the active surface of the HMF antenna 104. Asnoted above, the return wire segments 110 of wires 102 complicate theexact magnetic field geometry of the HMF antenna 104, but close to thecenter of the active surface of the HMF antenna 104, the horizontalcomponent of the magnetic field dominates the other components.

[0058] A preferred embodiment of the sensor system is described. Thecurrent in the HMF antenna 104 is controlled by the high speedE-Switches 106 operating in parallel. The E-Switches 106 are electronicrelays with very fast turn-off times, preferably constructed usinginsulated gate, bipolar transistors in a floating configuration. Oneskilled in the art can construct the E-Switch 106. Opto-isolators areused to couple the E-Switches 106 to the ground-referenced pulse controlcircuitry 112.

[0059] The HMF magnetic field sensor system 100 is preferably configuredso that a single power supply 114 provides current to the HMF antenna104. It is understood that a different power supply arrangement could beused that satisfied specific application requirements. For example, thepresent embodiment requires a relatively high voltage and high currentpower supply to drive the chain of E-Switches 106. Multiple lowervoltage and/or lower current power supplies could be employed that havethe same desired magnetic field characteristics. Also, the powersupplies could be configured so that different current is flowingthrough different portions of the HMF antenna wires. Using differentcurrent levels in different parts of the HMF antenna 104 allows one totailor the spatial character of the magnetic field.

[0060] The E-Switches 106 turn-off the antenna current in less thanabout 400 ns (90% to 10% amplitude). To reduce the number of E-Switches106, each E-Switch 106 controls four closely spaced parallel wires 102.It is understood that the number of wires controlled by the E-Switches106 is only limited by the exact electrical characteristics of theE-Switches 106 and the electrical properties of the wires (e.g.,resistance, inductance and capacitance). More or less wires could beconnected to the E-Switches 106 depending on the application.

[0061] Two banks 116 (see FIG. 8) of 16 E-Switches 106 control 64parallel wires 102 forming the active surface of the HMF antenna 104.The size of the HMF antenna 104 is approximately 80 cm by 180 cm and theparallel wires 102 are spaced about 1.3 cm apart.

[0062]FIG. 9 is a diagram illustrating a wiring configuration of the HMFantenna 104 having twisted parallel wires 102, twisted return wires 110and damping resistors 146. FIG. 9 only shows one twisted parallel wire102 and one twisted return wire 102 for clarity. It is to be appreciatedthat all the parallel wires 102 and all return wires 110 are twisted.E-Switches 106 are provided to control the antenna current flow. Whenthe E-Switch 106 goes from a closed position (current flowing in loop)to an open position, a damping resistor 146 placed across the loopdampens the current as quickly as possible without circuit oscillationor a long decay time (critically damped).

[0063] The inventive arrangement of wires and damping resistors 146minimizes circuit oscillation, minimizes the time for current dampingand minimizes the magnetic field collapse time. In a time-domain metaldetection sensor, the sooner the primary excitation magnetic fielddisappears, the sooner the small eddy currents from the target can bemeasured. The decay of the primary magnetic field is governed by thedecay of the current in the wire after the switch is opened.

[0064] Ignoring capacitive effects, the decay time on the loop is givenby L/R, where L is the inductance of the loop and R is the resistance ofthe loop. Minimizing L and making R as large as possible without causingoscillation minimizes the decay of the current in the loop andtherefore, the presence of the primary excitation magnetic field.Breaking the loop up into segments as shown in FIG. 9 reduces the L ofthe segment. Having a return wire 110 between the two switches 106allows a damping resistor 146 to be placed across the wire segment,therefore, damping the inductance of that wire segment.

[0065]FIG. 9 shows two damping resistors 146. Although not necessary forthe operation of the damping function, using two resistors 146 addsymmetry to the damping function. In wide-bandwidth circuits, symmetriccircuit layouts work more effectively. The HMF antenna arrangement alsoworks with just one E-Switch 106 controlling the section of wires, butless effectively. Additionally, a single resistor could be used for allthe wires controlled by the E-Switch 106.

[0066] III. Experimental Results

[0067] III.1 Experimental Setup

[0068] Experiments were conducted to validate the operation of the HMFsensor system. FIG. 8 shows a diagram of the experimental setup (topview) for the test target measurements. Two 15 cm by 15 cm, 16-turnprinted circuit board receiver coils 120 were placed near the center ofthe HMF antenna 104. The receiver coils 120 were connected in adifferential arrangement to subtract any residual coupled antenna decaycurrent and far-field electrical noise.

[0069] Test targets 122 were placed directly over one edge of thereceiver coil 120 as shown in FIG. 8. The receiver signal was amplifiedby a wide-bandwidth, multi-stage differential amplifier (not shown). Theoutput of the amplifier was digitized with a data acquisition systemmounted in a personal computer.

[0070] III.2 Target Responses

[0071] To validate the sensor system's ability to accurately measuretime decay responses from metal targets, the sensor system 100 wastested with a calibration loop. A thin-wire loop can be modeled with asingle exponential decay parameter that can be calculated analyticallyfrom theory.

[0072] A calibration loop was formed from a single turn of #22AWG(American Wire Gage) copper wire with a diameter of 10.1 cm. The loopwas placed in three orientations over the sensor system 100: the axisparallel to the plane of the HMF antenna 104 (i.e., maximum coupling toBx flux) at z=10 cm; the axis 45 degrees to the plane of the HMF antenna104 with center of loop at z=10 cm; and the axis perpendicular to theplane of the HMF antenna 104 with center of loop at z=10 cm.

[0073]FIG. 10 shows time decay response data from the calibration testover the time period of 8 μs to 200 μs. A nonlinear least-squares methodwas used to fit the wire loop time decay response data to a singleexponential-term equation over the time range of 10 μs to 50 μs. Thecalculated time decay of the wire loop is 20.4 μs and compares favorablywith the measure time decay of 20.6 μs for both the parallel and 45degree orientations. The results of the time constant calibration givesconfidence that the sensor system 100 measures accurate target timedecay responses.

[0074]FIG. 10 shows three features of the target responses as measuredby the sensor system 100:

[0075] First, while the amplitude of the time decay response may changedue to the different flux coupling between the HMF antenna 104 andreceiver unit 120, the time decay does not. The 0° and 45° time decayresponses are the same.

[0076] Second, when the loop target axis is perpendicular to the HMFantenna 104, the response is approximately zero. When the calibrationloop is oriented in this fashion, there is approximately zero fluxcoupling from the HMF antenna 104 into the loop. Therefore, there is noflux change to excite eddy currents. This result validates the idea thatthe primary magnetic field component, at this location over the HMFantenna 104, is horizontal.

[0077] Third, target time decay response can be measured relativelyclose to the antenna turn-off time.

[0078] The time decay data in the time region between 0 μs and 8 μs isnot shown in FIG. 10, since the receiver unit 120 was either insaturation or was oscillating. FIG. 11 shows some of these sensorartifacts in the time region of 5 μs to 8 μs. It is contemplated to usea higher bandwidth and wider dynamic range magnetic field antenna tomeasure target time decay closer to the antenna turn off time. Thiswould enable the detection of low metal content targets, since suchtargets have a fast time decay response. Since the HMF antenna turns offin less than 1 μs, the performance (i.e., increased bandwidth) of thesensor system can be improved by using higher frequency receiver coilsor higher bandwidth magnetic sensors.

[0079] The primary objective of the HMF sensor system 100 is targetidentification and classification based on eddy current time decay. Withthis objective in mind, FIG. 11 presents preliminary time decay responsedata from two different targets at two different orientations to the HMFantenna plane. The absolute amplitude of the target time decay responsedoes not govern the target characterization process. Therefore, in orderto show more clearly the differences in the relative time decayresponses of different targets with large differences in absoluteamplitude, the time decay data amplitudes have been normalized to 1 at15 μs. FIG. 11 is a log-log plot of data from an aluminum soda can and aVal 59 metal anti-personnel (AP) mine taken approximately 15 cm abovethe plane of the HMF antenna 104. The two targets were oriented bothvertically and horizontally relative to the plane of the HMF antenna104. FIG. 11 clearly shows that the two targets can be differentiatedfrom each other based solely on their time decay responses. This targetdifferentiation has been demonstrated in other research and forms thebasis of target identification and classification. FIG. 11 demonstratesthat the sensor system 100 has the capability to measure target timedecay responses that are different for different targets, and foridentifying the different targets.

[0080] III.3 Operation

[0081] A description will now be provided as to the operation of the HMFantenna 104 with different receiver configurations. These receiverconfigurations can be used singly or in combination. We start withsimple configurations that show the underlying concepts of the HMFsensor system features and move to more complex receiver configurationsthat take full advantage of the HMF antenna characteristics.

[0082]FIG. 12A is a side view diagram illustrating operation of the HMFantenna 104 where the receiver units 120 a, 120 b are verticallyoriented. FIG. 9B is a top view diagram illustrating a vertical receiverarray configuration for the receiver units 120 a, 120 b of FIG. 9A.Vertically oriented receivers refers to the axis of sensitivity of thereceiver, i.e., the receiver is sensitive to vertical magnetic fields.The receiver units 120 are located near the opposite edges of the HMFantenna 104 above the plane of the HMF antenna 104 (toward the directionof the metal target 140) and are connected to differential amplifier121. Even though the receiver units 120 are illustrated as using aninduction coil, it is contemplated that the receiver units 120 can useother types of magnetic sensors.

[0083]FIG. 12A shows the metal target 140 located near the centerline ofthe HMF antenna 104 at some distance from the plane of the HMF antenna104. The HMF antenna 104 generates a magnetic field in the positivex-direction as indicated by the HMF excitation arrow pointing to theright. When the magnetic field is turned off, the metal target 140responds by generating a magnetic field that opposes the collapsingmagnetic field, i.e., eddy currents in the metal target 140 aregenerated. FIG. 12A shows conceptually the magnetic field (targetresponse) generated by the metal target's eddy currents. At the leftreceiver unit 120 a, the magnetic flux from the target response isdownward in the −z direction, (−Bz). The vertical (z sensitive) magneticfield receiver unit 120 a detects this magnetic flux and sends thesignal VI to a differential amplifier's negative input. The differentialamplifier is designated by reference numeral 144.

[0084] At the same time, the right receiver unit 120 b intercepts themagnetic flux from the target response which is upward, +z direction,(+Bz). The nominally identical right vertical magnetic field receiverunit 120 b detects this magnetic flux and sends the signal V₂ to thedifferential amplifier's positive input. The output from thedifferential amplifier 144 can be written as:

[0085] V=V₂−V₁+N₂−N₁

[0086] where V₁ is the left receiver unit signal, V₂ is the rightreceiver unit signal, N₁ is the electromagnetic (EM) noise (e.g.,interference) measured by the left receiver unit 120 a and N₂ is the EMnoise measured by the right receiver unit 120 b. Since the EM noise seenby both receiver units 120 a, 120 b is nominally the same and V₁≈−V₂,the output of the differential amplifier 144 is then,

[0087] V=2*V₂

[0088] The EM noise terms cancel and we double our target signal. Theamplification of the target signal is dependent on the spatialrelationship between the target 140 and the sensor system 100. However,the EM noise cancellation is still effective and allows the sensorsystem 100 to operate in EM noisy environments.

[0089] It is also obvious that the receiver units 120 could be wiredtogether directly to create the same effect as that generated by thedifferential amplifier 144. This alternate connection of the receiverunits 120 is understood in the following discussion. The use of thedifferential amplifier 144 makes the discussion of signal addition andsubtraction clear.

[0090]FIG. 13A is a side view diagram illustrating operation of the HMFantenna where the receiver units 120 a, 120 b are horizontally oriented.FIG. 13B is a top view diagram illustrating a horizontal receiver arrayconfiguration for the receiver units 120 a, 120 b of FIG. 13A. Referringback to FIG. 13A, the receiver units 120 a, 120 b are nominally locatedalong the centerline of the HMF antenna 104. When the target 140 islocated directly under the plane of the HMF antenna 104, the target fluxis primarily horizontal at the plane of the antenna 104. The horizontalreceiver units 120 a, 120 b detect this flux.

[0091]FIG. 13A shows the HMF antenna 104 generating a magnetic field inthe negative x-direction as indicated by the HMF excitation arrowpointing to the left. When the magnetic field is turned off, the metaltarget 140 responds by generating a magnetic field that opposes thecollapsing magnetic field, i.e., eddy currents in the metal target 140.FIG. 13A shows conceptually the magnetic field (target response)generated by the metal target's eddy currents. The target response fluxat the plane of the HMF antenna 104 is pointing to the left as denotedby the small arrows. The signals from the two receivers units 120 a, 120b are preferably summed in an amplifier 145. The signal measured by thereceiver units 120 a, 120 b can be written as:

[0092] Signal Output=R1(T)+R1(N)+R1(A)+R2(T)+R2(N)+R2(A).

[0093] R1 is the top receiver unit 120 a and R2 is the bottom receiverunit 120 b, R1(T) is the top receiver unit's target signal, R1 (N) isthe top receiver unit's EM noise signal, R1(A) is the top receiverunit's antenna signal, R2(T) is the bottom receiver unit's targetsignal, R2(N) is the bottom receiver unit's EM noise signal and R2(A) isthe bottom receiver unit's antenna signal. If the two receiver units 120a, 120 b are placed symmetrically on the antenna 104, then R1(A)=−R2(A)and the antenna signals cancel when added together by the summingamplifier 145. One is then left with approximately twice the targetsignal and twice the EM noise signal.

[0094] FIGS. 14-15B illustrate how to reconfigure the receiver units 120a, 120 b in FIG. 13A for both antenna flux cancellation andadditionally, flux cancellation from EM noise and the eddy currentsgenerated in the ground (ground response). The feature of ground eddycurrent cancellation is called ground-balancing and is an importantconcept for low-metal target and underground void detection.

[0095]FIG. 14 is a diagram illustrating the horizontal receiver arrayconfiguration having the horizontal receiver units 120 mounted in adifferential mode. Receiver unit 1 is mounted on the top side of the HMFantenna 104 and receiver unit 2 is mounted on the bottom side of the HMFantenna 104 as in FIG. 13A. The signals of the two receiver units 1, 2are summed together and the output is sent to the plus input ofdifferential amplifier 148. Additionally, a second set of identicalreceiver units 3, 4 are located near the first set, but not directlyover the target under study. Their summed output is sent to the negativeinput of the differential amplifier 148. Using similar notation asabove, the output signal of the differential amplifier 148 can bewritten as:

[0096] A=R1(T)+R1(N)+R1(A)+R1(G)+R2(T)+R2(N)+R2(A)+R2(G)

[0097] B=R3(N)+R3(A)+R3(G)+R4(N)+R4(A)+R4(G).

[0098] Where R1 and R3 are the two top receiver units 1, 2 and R2 and R4are the two bottom receiver units 3, 4; R1(T), R1(N), R1(A), and R1(G)are the first receiver unit's target, EM noise, antenna and groundsignals, respectively; R2(T), R2(N), R2(A), and R2(G) are the secondreceiver unit's target, EM noise, antenna, and ground signals,respectively; R3(N), R3(A), and R3(G) are the third receiver unit's EMnoise, antenna, and ground signals, respectively; and R4(N), R4(A), andR4(G) are the fourth receiver unit's EM noise, antenna, and groundsignals, respectively.

[0099] If the top two pairs of receiver units 1, 2 are placedsymmetrically on the antenna 104, then:

[0100] R1(T)≈R2(T)

[0101] R1(A)=R3(A)=−R2(A)=−R4(A)

[0102] R1(N)=R3(N)=R2(N)=R4(N).

[0103] The differential amplifier 148 (or alternatively, differencingvia direct connection with wires in reverse order as in the case ofcounter-wound induction coil receivers) subtracts B from A and:

[0104] Output=A−B

[0105] Output=2*R1(T)+R1(G)+R2(G)−R3(G)−R4(G).

[0106]FIG. 15A shows the case of a medium or large metal target 140 thathas a response that is larger than the ground response. The relativesize of the arrows in FIG. 15A indicate the relative response's from themetal and ground. Then,

[0107] Output=2*R1(T)R(G)<<R(T).

[0108] Hence, the medium or large metal target 140 is detected by thesystem 100 and/or an operator.

[0109] For the case of a small metal target where the metal response isnot large with respect to the ground, then,

[0110] R1(G)≈R3(G)

[0111] R2(G)≈R4(G)

[0112] Output≈2*R1(T).

[0113] The case of an underground void is shown by FIG. 15B, where theunderground void is designated generally by reference numeral 142. Theground response of the underground void 142 is less than the groundresponse without the underground void 142. When the signals aresubtracted in differential amplifier 151, the void signal is negative.Accordingly, the system 100 and/or the operator detect the existence ofthe underground void 142.

[0114] If there is an underground void 142 and a small metal contenttarget present, as is the case for a low-metal content mine, the timedecay signal will be composed of both negative (void) and positive(metal) signals. The eddy current time decay of a void and metal targetare very different. Accurately measuring the time decay history of thetarget response allows one to separate the void and metal signals. Theexistence of a coincident metal and void signal is an indication of alow-metal content mine.

[0115] IV. Forming the Steerable 3-D Magnetic Field Sensor System

[0116] One method to model a metal target is to define a magneticpolarizability tensor $\begin{matrix}{\overset{->}{M} = {\begin{pmatrix}{M_{x}(t)} & 0 & 0 \\0 & {M_{y}(t)} & 0 \\0 & 0 & {M_{z}(t)}\end{pmatrix}.}} & (3)\end{matrix}$

[0117] where the diagonal components of the tensor are the timeresponses of the target to excitations in an orthogonal reference framecentered on the target. For a loop antenna oriented directly over atarget, the antenna only excites the vertical component of the target'stime decay response. For accurate target classification, it is desirableto measure all three components of a target's magnetic polarizabilitytensor. Accordingly, a discussion will now be presented for combiningtwo single HMF sensor systems 100 to form a steerable two-dimensional(2-D) HMF sensor system and then combining a steerable 2-D HMF sensorsystem with a vertical loop antenna sensor system and forming thesteerable 3-D magnetic field sensor system.

[0118] With reference to FIGS. 16 and 17, there are shown two HMFantennas 104 a, 104 b at right angles to each other forming atwo-dimensional HMF antenna 150 that can generate a horizontal magneticfield which can be steered in any direction in the plane of the antennas104 a, 104 b, not just the direction perpendicular to the current flowin the antenna wires 102, and a diagram of a steerable HMF sensor system200 having the two-dimensional HMF antenna 150, respectively. FIG. 16shows one wire 102 a representing x-direction HMF antenna 104 a withcurrent flow Ix in the x-direction and one wire 102 b representingy-direction HMF antenna 104 b with current flow Iy in the y-direction.

[0119] By controlling the current separately in each HMF antenna 104 a,104 b using current control circuitry 160 a, 160 b under computercontrol 170 (FIG. 17), one can create a new magnetic field pointed inany direction in the plane of the HMF antennas 104 a, 104 b. The newmagnetic field is given by B=(Bx²+By²)^(1/2). The angle of the field isgiven by θ=tan⁻¹[By/Bx]. It is provided that receiver units 120 areprovided adjacent each of the HMF antennas 104 a, 104 b as describedabove with reference to FIGS. 8 and 12A-15B.

[0120] One skilled in the art would appreciate that additional HMFantennas 104 may be provided to the system 200.

[0121]FIG. 18 is a schematic diagram of the steerable 3-D magnetic fieldsensor system 300 according to the present invention. The sensor system300 adds to the 2-D steerable HMF sensor system 150 a vertical loopantenna 180 which adds the third dimension to the steerable magneticfield sensor system. Accordingly, the generated magnetic field of thesensor system 300 is a summation of the magnetic field in the x-axisdirection generated by the HMF antenna 104 a, the magnetic field in they-axis direction generated by the HMF antenna 104 b, and the magneticfield in the z-axis direction generated by the vertical loop antenna180.

[0122] At least one vertical magnetic field receiver (not shown) isunderstood to be included in the sensor system 300. A z-antenna currentcontrol circuitry 160 c is also provided which is under computer control170 for controlling the magnetic field in the z-direction.

[0123] It is contemplated to provide a computer data collection systemto the sensor system 100 for digitizing the time decay data from theoutput of the receivers 120. The data would then be analyzed to optimizethe system's operating parameters, such as antenna current, digitizersample rate and time sampling window, for optimal target data collectionand characterization. Once a target's time decay response is measured,the target can then be classified and identified using a matched filteror other classification/identification approach. It is furthercontemplated to configure the steerable magnetic field sensor system 100for mounting to a vehicle or aircraft to provide a vehicle mounted orairborne mine detector sensor system.

[0124] It is further contemplated to configure the HMF antenna 104, thesteerable 2-D or 3-D magnetic field sensor systems for mounting to avehicle to provide a vehicle mounted mine/UXO detector sensor system;for detection, localization and identification of buried utilities; andfor detection and identification of hidden targets at critical points ofentry, such as airports and secured areas. The antenna and systemsconfigured could also be for mounting to an aircraft or other airborneplatform for airborne active electromagnetic surveys for mines, andburied ordnance and utilities.

[0125] What has been described herein is merely illustrative of theapplication of the principles of the present invention. For example, thefunctions described above and implemented as the best mode for operatingthe present invention are for illustration purposes only. Otherarrangements and methods may be implemented by those skilled in the artwithout departing from the scope and spirit of this invention.

1. A sensor system for inducing eddy currents in objects comprising: anantenna configured for generating a magnetic field in a directionperpendicular to current flow in a plurality of wires along a plane ofthe antenna; at least two receiver units in proximity to the plane ofthe antenna, each of said two receiver units configured for convertingthe eddy currents into a first signal and a second signal; and anamplifier for receiving the first and second signals.
 2. The systemaccording to claim 1, wherein the antenna has a rectangularconfiguration having a length of approximately 180 cm and a width ofapproximately 80 cm.
 3. The system according to claim 1, wherein theantenna includes a first bank and a second bank of switches, the firstbank of switches is at one end of the antenna and the second bank ofswitches is at an opposite end of the antenna, and the first bank ofswitches and the second bank of switches are connected by a plurality ofreturn wire loops and the plurality of wires along the plane of theantenna.
 4. The system according to claim 1, wherein adjacent wires ofthe plurality of wires along the plane of the antenna are equally-spacedand parallel with respect to each other.
 5. The system according toclaim 1, wherein the amplifier subtracts the first signal from thesecond signal.
 6. The system according to claim 1, wherein each of theat least two receiver units measures approximately 15×15 cm and has 16loop turns.
 7. The system according to claim 3, wherein the first bankand the second bank of switches are controlled by pulse controlcircuitry.
 8. The system according to claim 3, wherein each of theplurality of return wire loops includes a wire segment perpendicular tothe equally-spaced parallel wires.
 9. The system according to claim 3,wherein the first bank and the second bank of switches each include 16switches.
 10. The system according to claim 9, wherein fourequally-spaced parallel wires of the plurality of wires couple each ofthe switches of the first bank of switches with a respective switch ofthe second bank of switches.
 11. The system according to claim 1,wherein the at least two receiver units are provided above the plane ofthe antenna.
 12. The system according to claim 1, wherein the at leasttwo receiver units are provided below the plane of the antenna.
 13. Thesystem according to claim 1, where one of the at least two receiverunits is placed below the plane of the antenna and the other of the atleast two receiver units is placed above the plane of the antenna. 14.The system according to claim 1, wherein the amplifier adds the firstsignal and the second signal.
 15. The system according to claim 3,wherein each of the plurality of return wire loops include two wiresconnected by at least one damping resistor, and wherein each of theplurality of return wire loops include a twisted portion.
 16. The systemaccording to claim 1, wherein each of the wires along the plane of theantenna is twisted with an adjacent wire and both wires are connected byat least one damping resistor.
 17. The system according to claim 1,further comprising another antenna oriented at approximately 90 degreeswith respect to the antenna for generating another magnetic field in adirection perpendicular to the current flow in a plurality of wiresalong its plane.
 18. The system according to claim 17, furthercomprising current control circuitry for controlling the direction of amagnetic field which is a summation of the magnetic fields generated bythe two antennas.
 19. The system according to claim 17, furthercomprising an antenna encircling the two antennas for generating anothermagnetic field.
 20. The system according to claim 19, further comprisingcurrent control circuitry for controlling the direction of a magneticfield which is a summation of the magnetic fields generated by the threeantennas.
 21. A sensor system comprising: an antenna having a pluralityof wires along a plane thereof for generating a magnetic field; andcontrol circuitry configured to steer the magnetic field in a pluralityof directions, including exclusively in a direction perpendicular to thecurrent flow in the plurality of wires.
 22. The system according toclaim 21, wherein the antenna includes a first bank and a second bank ofswitches, the first bank of switches is at one end of the antenna andthe second bank of switches is at an opposite end of the antenna, andthe first bank of switches and the second bank of switches are connectedby a plurality of return wire loops and the plurality of wires.
 23. Thesystem according to claim 21, wherein the plurality of wires areequally-spaced with respect to each other.
 24. The system according toclaim 22, wherein the plurality of return wire loops include two wiresconnected by at least one damping resistor, and wherein each of theplurality of return wire loops include a twisted portion.
 25. The systemaccording to claim 21, wherein each of the plurality of wires is twistedwith an adjacent wire and both wires are connected by at least onedamping resistor.
 26. The system according to claim 21, wherein theantenna includes a first antenna and a second antenna oriented atapproximately 90 degrees with respect to each other, and wherein themagnetic field is a summation of a magnetic field generated by the firstantenna and a magnetic field generated by the second antenna.
 27. Thesystem according to claim 26, wherein the antenna includes a thirdantenna encircling the first and second antennas, and wherein themagnetic field is a summation of the magnetic field generated by thefirst antenna, the magnetic field generated by the second antenna, and amagnetic field generated by the third antenna.
 28. An antenna systemcomprising: a first antenna configured for generating a magnetic fieldin the x-axis direction; a second antenna configured for generating amagnetic field in the y-axis direction and oriented at a 90-degree anglewith respect to the first antenna; and a third antenna configured forgenerating a magnetic field in the z-axis direction and encircling thefirst and second antennas.
 29. The system according to claim 28, furthercomprising current control circuitry for controlling the direction of amagnetic field which is a summation of the magnetic field in the x-axisdirection, the magnetic field in the y-axis direction, and the magneticfield in the z-axis direction.
 30. The system according to claim 28,wherein the first and second antennas include a plurality of wires, andwherein adjacent wires are connected to each other by at least onedamping resistor.